The acetone-butanol fermentation, known for a long time, results in the conversion of various sugars to a mixture of industrial interest: butanol, acetone and ethanol (ABE); it thus constitutes a particularly advantageous process for obtaining the main product: n-butanol. The advantage of this manner of obtaining components of alternate fuels has also been disclosed in the French patent application Nos. 80/12822 and 80/17147 of Institut Francais du Petrole. The main limitation of this fermentation has however been, up to now, the relatively low concentration of the resultant products which is generally not higher, under the best conditions, than a total of 20 g/l, typically 10-13 g/l of n-butanol, 4-7 g/l of acetone and 0-2 g/l of ethanol, as shown in the synthesis publications of M. T. Walton and J. L. Martin (Microbial Fermentation, 2nd edition, vol. 1, Academic Press, 1979, p. 187-209) and S. C. Prescott and C. G. Dunn (Industrial Microbiology, Mc Graw Hill, 1959, p. 250-293).